1. Field of the Invention
The present invention relates to a copying machine capable of changing a copy magnification and, more particularly, to an exposure control apparatus for controlling an amount of light to be radiated from an exposure lamp onto a document in accordance with a copy magnification.
2. Description of the Related Art
Japanese Patent Disclosure (Kokai) No. 60-17438 discloses an exposure control apparatus for controlling an amount of light to be radiated from an exposure lamp onto a document in a copying machine.
This exposure control apparatus includes a reference voltage generating circuit for controlling an exposure amount. This circuit has a digital attenuator. The exposure control apparatus obtains an optimal exposure amount corresponding to a given copy magnification by adjusting the digital attenuator on the basis of data corresponding to the copy magnification.
A conventional copying machine is designed to change copy magnifications from reduction (magnification: 71%) to enlargement (magnification: 141%) in a stepless manner. For this reason, when exposure amounts of an exposure control apparatus are to be set, exposure amounts are adjusted at, e.g., three magnification points; one-to-one magnification (magnification: 100%), reduction (magnification: 71%), and enlargement (magnification 141%), and a curve is obtained by connecting points representing the exposure amounts set at these three points. Since this curve represents exposure amounts corresponding to the copy magnification at the three points and other copy magnifications, an exposure amount corresponding to a predetermined copy magnification can be obtained from this curve.
When a document is actually copied, an exposure amount corresponding to a given copy magnification is obtained from the above-described curve, and a digital attenuator of the exposure control apparatus is adjusted in accordance with the obtained exposure amount. Therefore, even if magnifications are changed from enlargement to reduction in a stepless manner, optimal exposure amounts can be set.
A copying machine capable of setting copy magnifications from 65% to 200% has recently been developed. Similar to the above-described copying machine, in this copying machine, exposure amounts are adjusted on the basis of only the three points, i.e., one-to-one magnification, enlargement, and reduction. In this copying machine, however, since the range of copy magnifications from one-to-one magnification to enlargement is wide, optimal exposure amounts cannot be obtained near a magnification of 140%.